Serum inhibin was measured by radioimmunoassay in 19 women undergoing ovulation induction within an in vitro fertilization program. In 16 nonpregnant subjects, inhibin rose during the luteal phase to reach a peak (2.5 U/ml) 6 days after oocyte retrieval; it then fell to undetectable levels by day 14 (less than 0.37 U/ml). Serum inhibin was significantly correlated (P less than 0.001) with both serum progesterone and estradiol concentrations and inversely correlated with serum follicle-stimulating hormone (FSH) (P less than 0.001). In the pregnant group (n = 3), inhibin levels reached an initial peak (3.7 U/ml) on day 6, then declined to a nadir (1.5 U/ml) on day 8 after oocyte retrieval and subsequently rose to 3.4 U/ml on day 14. These data suggest (1) an inverse relationship between inhibin and FSH, which is in support of the inhibin hypothesis; (2) that the human corpus luteum secretes inhibin; and (3) a trophoblastic/placental origin of inhibin during gestation.